(a) Field of the Invention
The present invention relates to a semiconductor laser device having a current non-injection area and, more particularly, to a semiconductor laser device suitably used for recording data on a recording medium such as an optical disk and a magneto-optical disk.
(b) Description of the Related Art
Red-ray semiconductor laser devices having higher output powers, such as an AlGaInP-group semiconductor laser device, are generally used for recording data on a recording medium, such as a DVD (digital versatile disk) or a magneto-optical disk. JP-A-4-218993 describes a self-aligned-structure (SAS) semiconductor laser device as a high-output-power laser device.
Referring to FIG. 19, the semiconductor laser device includes an n-type GaAs substrate (referred to as n-GaAs substrate hereinafter) 101, and a layer structure including a buffer layer 102, n-type lower cladding layers 103 and 104, an active layer 105 and a p-type upper cladding layer 106, a current blocking layer 121, a cap layer 109, an external cladding layer 110 and a p-side electrode 111, which are consecutively formed on the n-GaAs substrate 101. The current blocking layer 109 and the cap layer 109 have a stripe opening, through which the external cladding layer 110 contacts the p-type cladding layer 106.
It is known that this type of the semiconductor laser device has improved characteristics such as a lower threshold current and a higher lasing efficiency by using AlInP or AlGaInP in the cladding layers (or optical confinement layers). This is because AlInP and AlGaInP scarcely absorb laser.
However, there is a problem in the semiconductor laser device having such a higher output power that it is susceptible to a catastrophic optical damage (COD) in a higher output power range of the laser device to have a defect on the facet of the laser cavity.
JP-A-02-239679 describes a semiconductor laser device which is capable of preventing the COD by providing a current blocking layer, which prevents current injection in the vicinity of the facet. However, there arises another problem in that this type of the laser device does not effectively confine the laser in the lateral direction in the vicinity of the facet, thereby causing an unstable lateral mode.
JP-A-2001-196693 describes a SAS semiconductor laser device which is capable of solving the above problems and preventing the COD by diordering the vicinity of the emission facet of the laser cavity to thereby suppress the laser absorption at the emission facet. However, there arises another problem in that this type of the laser device has a larger leakage current in the vicinity of the facet to thereby degrade the laser characteristics as to the threshold current and the slope efficiency.
JP-A-2001-332811 describes a semiconductor laser device which is capable of solving the above problems by controlling the thickness of the current blocking layer in the vicinity of the cavity facet, thereby providing a current non-injection area in the vicinity of the cavity facet. This structure achieves suppression of the COD, and allows the lateral mode and the radiation angle of the laser device to be effectively controlled. However, it is generally difficult to control the thickness of the current blocking layer within the area thereof. In addition, there is a problem in that the difference in the profile of the refractive index between the current injection area and the current non-injection area raises the mode dispersion loss of the laser device.
JP-A-62-51281 describes a semiconductor laser device which is capable of suppressing the COD by using two current blocking layers, wherein the two current blocking layers have aligned openings to form a current injection area and the lower current blocking layer has no opening in the vicinity of the facet to form a current non-injection area in the vicinity. This structure is free from the problem of the difficulty in the thickness control of the current blocking layer. However, there remains the problem of the difference in the profile of the refractive index between the current injection area and the current non-injection area, causing the increase of the mode dispersion loss.